The aim of the present invention is to provide a plant for the conversion of at least one cut selected from the group formed by olefinic C.sub.4 cuts and olefinic C.sub.5 cuts to an ether (an alkyl-tertiobutylether or an alkyl-tertioamylether) and to propylene. When the cut originates from a steam cracking operation, a further aim of the present invention is to optimise the relative ethylene-propylene selectivity of the steam cracking procedure using this plant.
Steam cracking of feeds constituted by light paraffin cuts supplies ethylene and propylene for the petrochemical industry. It also provides a number of other heavier products, in particular a C.sub.4 hydrocarbon cut which contains mainly 1,3-butadiene, isobutene, n-butenes and butanes, accompanied by traces of acetylenic hydrocarbons, and a C.sub.5 hydrocarbon cut which contains mainly C.sub.5 diolefins, methylbutenes, n-pentenes and pentanes, accompanied by traces of acetylenic hydrocarbons.
As well as the principal products of petrol and gas oil fractions, catalytic cracking of heavy hydrocarbon feeds, in particular fluid catalytic cracking (FCC), produces lighter products, among them a C.sub.4 hydrocarbon cut which contains mainly isobutane, isobutene, n-butenes and butanes, accompanied by small quantities of 1,3-butadiene and acetlyenic hydrocarbons, and a C.sub.5 hydrocarbon cut which contains mainly pentanes, methylbutenes and n-pentenes, accompanied by small quantities of C.sub.5 diolefins and acetlyenic hydrocarbons.
Until recently, only 1,3-butadiene and isobutene were of use in the polymer industry, in particular in the tire industry for the former. An increase in tire lifetime and a relative stagnation of demand has led to a surplus of butadiene which is not used to any great extent. In contrast, isobutene has gained in importance as it can be used to synthesize ethers which are used as additives for motor fuels.
The present invention provides a plant for the treatment of a C.sub.4 hydrocarbon cut containing mainly isobutene, n-butenes, butanes and varying amounts of 1,3-butadiene, which includes the transformation of isobutene to ethers (for example MTBE), and which can transform 1,3-butadiene and n-butenes to propylene for polymerisation, for example. It also provides a plant for the treatment of a C.sub.5 cut to transform it primarily into ethers (for example TAME) and propylene. It further provides a plant for the combined treatment of a C.sub.4 cut and a C.sub.5 cut.
The relative proportions of ethylene and propylene produced in a steam cracking operation can be modulated to a certain extent by changing the nature of the feed and modifying the cracking conditions (severity). However, one operating mode intended to produce a higher proportion of propylene inevitably produces larger quantities of C.sub.4 Cut, C.sub.5 cut and heavier petrol fractions of poor quality.
A further aim of the present invention is to optimise the relative ethylene-propylene selectivity of the steam cracking procedure by treating the C.sub.4 or C.sub.5 steam cracking hydrocarbon cuts to produce, inter alia, propylene, thus enabling the relative proportions of ethylene and propylene to be adjusted without being obliged to change the cracking severity.
More precisely, the object of the invention is to provide a plant for the conversion of at least one olefinic cut, selected from the group formed by a C.sub.4 cut and a C.sub.5 cut, to an ether and to propylene, the cuts containing mainly diolefins, alpha olefins, isoolefins and acetylenic impurities the already patented process comprises the following steps which are carried out successively:
1) selective hydrogenation of the diolefins and acetylenic impurities with simultaneous isomerisation of the alpha olefins to internal olefins by passing said cut in the liquid phase over a catalyst comprising at least one metal selected from the group formed by nickel, palladium and platinum, deposited on a support, at a temperature of 20-200.degree. C., a pressure of 1-5 MPa, and a space velocity of 0.5-10 h.sup.-1, with a H.sub.2 /diolefin (molar) ratio of 0.5 to 5, preferably 1 to 3, to obtain an effluent containing mainly internal olefins and isoolefins, and containing practically no diolefins or acetylenic compounds; PA1 2) etherification of the isoolefins by reacting the effluent from the preceding step with an alcohol, in the presence of an acid catalyst, at a temperature of 30-130.degree. C., at a pressure such that the reaction is carried out in the liquid phase, the ether and alcohol being separated simultaneously with or after etherification to obtain an effluent containing mainly internal olefins accompanied by oxygen-containing impurities; PA1 3) separation of the oxygen-containing impurities from the effluent from step 2); PA1 4) metathesis of the effluent from the preceding step with ethylene, in the presence of a catalyst comprising at least one rhenium oxide deposited on a support, at a temperature in the range 0.degree. C. to 100.degree. C., and at a pressure which is at least equal to the vapour tension of the reaction mixture at the reaction temperature, to obtain an effluent containing propylene, the metathesis being followed by separation of the propylene. PA1 selective hydroisomerisation of 1,3-butadiene with isomerisation of 1-butene to 2-butene and hydrogenation of acetylenic hydrocarbons; PA1 etherification with an alcohol to produce an alkyl-tertiobutylether; PA1 separation of oxygen-containing compounds; PA1 metathesis of the 2-butene-rich effluent in the presence of ethylene (ethenolysis) to produce propylene. PA1 selective hydroisomerisation of 1,3-butadiene to a mixture of n-butenes in thermodynamic equilibrium; PA1 isomerisation of 1-butene to 2-butene, also in thermodynamic equilibrium; PA1 hydrogenation of traces of acetylenic hydrocarbons.
The process aspect of the invention (illustrated in FIG. 1) will be described in more detail using a C.sub.4 hydrocarbon cut containing mainly isobutene, n-butenes, butanes, and varying amounts of 1,3-butadiene, supplied via a line 5. The C.sub.4 cut undergoes a succession of treatments which are consolidated in the following steps, to produce an alkyl-tertiobutylether and propylene:
The succession of treatments of the process of the invention has a number of advantages. The most reactive compounds in the cut, namely the diolefins (for example 1,3-butadiene) which are in varying amounts, and the traces of acetylenic hydrocarbons, are transformed in the first step and thus do not cause side reactions in the following steps. Further, selective hydroisomerisation of diolefins (for example 1,3-butadiene) in the steam cracking cuts can considerably increase the concentration of the corresponding internal olefin (2-butene in this case) in the cut, further enhancing the metathesis step and producing a high yield of propylene.
The principal aim of the first step is to transform 1,3-butadiene and n-butenes into 2-butene. 2-butene is the source of the propylene which is produced in the last metathesis step in the presence of ethylene. 1-butene does not produce a new product with the ethylene, and reacts with the 2-butene to produce propylene, but also to produce undesirable pentenes. Maximising the yield of 2-butene is thus desirable, i.e., to get as close as possible to the proportions dictated by thermodynamics. The second aim of this step is to eliminate the traces of acetylenic hydrocarbons which are always present in these cuts and which poison or pollute the subsequent steps.
In the first step (zone 1), the following reactions are carried out simultaneously, in the presence of hydrogen supplied via line 6:
These reactions can be carried out using various specific catalysts comprising one or more metals, for example from group 10 of the periodic classification (Ni, Pd, Pt), deposited on a support. Preferably, a catalyst is used which comprises at least one palladium compound fixed on a refractory mineral support, for example alumina. The amount of palladium on the support can be in the range 0.01% to 5% by weight, preferably in the range 0.05% to 1% by weight. A variety of known pretreatments may be applied to these catalysts to improve selectivity for the hydrogenation of 1,3-butadiene to butenes at the expense of complete hydrogenation to butane which must be avoided. The catalyst preferably contains 0.05% to 10% by weight of sulphur. Advantageously, the catalyst is constituted by palladium deposited on alumina and containing sulphur.
The catalyst can be sulphurized in situ (in the reaction zone) or preferably ex situ. In the latter case, the process described in French patent FR-93/09524 is preferably used, where the catalyst is treated, before being charged into the hydrogenation reactor, with at least one sulphur-containing compound diluted in a solvent, and the catalyst obtained containing 0.05% to 10% of sulphur (by weight) is charged into the reactor and activated in a neutral or reducing atmosphere at a temperature in the range 20.degree. C. to 300.degree. C., at a pressure in the range 0.1 to 5 MPa and at a GHSV in the range 50 to 600 h.sup.-1, where the feed is brought into contact with the activated catalyst.
The mode of use of the catalyst, which is preferably of palladium, is not critical, but it is generally preferred to use at least one downflow reactor with a fixed bed of catalyst. When the proportion of 1,3-butadiene in the cut is high, as is the case, for example, for a steam cracking cut when the 1,3-butadiene is not to be extracted for specific purposes, it may be of advantage to effect the transformation in two reactors in series to better control the hydrogenation selectivity. The second reactor may be an upflow reactor and may act as a finisher.
The quantity of hydrogen required for the reactions carried out in this step is adjusted as a function of the composition of the cut so that, advantageously, there is only a slight excess of hydrogen with respect to the theoretical stoichiometry.
The operating conditions are selected so that the reactants and products are liquid. It may, however, be of advantage to select an operating mode such that the products are partially vaporised at the reactor outlet, to facilitate thermal control of the reaction. The temperature can be between 20.degree. C. and 200.degree. C., preferably between 50.degree. C. and 150.degree. C., or more preferably between 60.degree. C. and 150.degree. C. The pressure can be adjusted to between 0.1 and 5 MPa, preferably between 0.5 and 4 MPa, advantageously between 0.5 and 3 MPa, so that the reactants are at least partially in the liquid phase. The space velocity can be in the range 0.5 to 10 h.sup.-1, preferably in the range 1 to 6 h.sup.-1, with a H.sub.2 S/diolefin (molar) ratio of 0.5 to 5, preferably 1 to 3.
Advantageously, the hydroisomerisation reactor or reactors is/are followed by a stabilizing column which eliminates traces of excess hydrogen and any methane.
The aim of the second step (zone 2) is to transform the isobutene present in the C.sub.4 cut from the preceding step into an alkyl-tertiobutylether by etherification with an alcohol having a hydrocarbon chain which can contain 1 to 10 carbon atoms, supplied via line 7. The alcohol is preferably methanol or ethanol. The ethers produced are respectively methyl-tertiobutylether (MTBE) and ethyl-tertiobutylether (ETBE), which leave via line 8.
Transformation is carried out using an acid catalyst, for example a catalyst based on an ion exchange resin in its H-form, such as a sulphonic acid resin--SO.sub.3 H. This catalyst can be used, for example, in a conventional fixed bed or in a mobile bed. An expanded catalyst bed is preferably used, maintained by an upflow of reaction medium through the reactor and an external heat exchanger. This type of operation can readily control the catalyst and eliminate the heat produced during, the reaction, by preventing the formation of hot spots.
A finishing reactor is advantageously located after the expanded bed reactor to maximise removal of isobutene from the residual cut and to increase the yield of ether, but the majority of the reaction occurs in the expanded bed reactor. A maximum amount of isobutene can also be removed by using reactive distillation in the finisher (a distillation column containing a catalyst) which can increase conversion by separating the products in the reactor.
The operating conditions are selected so that the reactants and products are in the liquid phase. In general, the temperature is fixed so that the reaction occurs at a sufficient rate, for example 30.degree. C. to 130.degree. C., preferably 40.degree. C. to 100.degree. C., and the pressure is adjusted as a consequence so that the reactants are in the liquid phase.
The reaction section is followed by a distillation section where the ether is separated at the bottom of the column from a distillate comprising the residual C.sub.4 cut and excess alcohol, also traces of other volatile oxygen-containing compounds. Referring to FIG. 6, the alcohol is separated by washing with water (W1) and the water-alcohol mixture is distilled (D4) to recycle the alcohol.
The C.sub.4 cut from the etherification step contains a small amount of a certain number of oxygen-containing and sulphur-containing compounds which must be separated in a third step (zone 3) of the process since these are poisons which reduce the efficiency of the metathesis catalysts.
A variety of methods can be used to carry out the separation. Preferably, the C.sub.4 cut from the etherification step is passed over a capture mass which has a higher affinity for oxygen-containing compounds than for olefinic or saturated hydrocarbons, and which can be constituted by refractory oxides and/or aluminosilicates which have an acidic nature, such as alumina, silica, silica-aluminas, or zeolites. When the etherification step uses methanol, it may be of advantage, in order to reduce the volume of the capture mass, to precede this step by a distillation column which separates the dimethylether which is a by-product of etherification and which dries the C.sub.4 cut at the same time.
The capture mass is used in a fixed bed, for example, and under temperature and pressure conditions such that the feed and the effluent are in the liquid phase. The temperature is in the range 0.degree. C. to 100.degree. C., preferably in the range 10.degree. C. to 50.degree. C. The pressure is in the range 0.3 to 5 MPa, preferably in the range 0.4 to 1 MPa. The space velocity is adjusted to obtain maximum capture of the oxygen-containing and sulphur-containing compounds.
When the capture mass is saturated with oxygen-containing and/or sulphur-containing compound, it must be regenerated, for example by entraining the adsorbed compounds in a hot gas stream. The gas can, for example, be nitrogen, methane or any gas which is inert towards the capture mass. The regeneration temperature can be in the range 200.degree. C. to 500.degree. C., preferably in the range 250.degree. C. to 450.degree. C. In order not to interrupt the continuous process, two capture masses can be used which operate alternately.
The majority of the cut obtained after the above succession of steps is constituted by butanes and 2-butene. In the final step (zone 4) of the process, the 2-butene is reacted with ethylene supplied via line 9 to produce propylene by metathesis (leaving via line 10). A line 11 evacuates the separated by-products.
The metathesis reaction of ethylene with 2-butene can be catalysed by various metal oxides deposited on supports. Preferably, a catalyst is used which comprises at least one rhenium oxide deposited on a support composed of a refractory oxide containing at least alumina, which has an acidic character, such as alumina itself, silica-aluminas or zeolites.
Preferred examples are catalysts comprising rhenium heptoxide deposited on a gamma alumina analogous to that used in reforming catalysts, as described in U.S. Pat. No. 4,795,734. The rhenium content (expressed as rhenium metal) can be in the range 0.01% to 20%, preferably in the range 1% to 15% by weight. The catalysts are, for example, subjected to final thermal activation at a temperature in the range 400.degree. C. to 1000.degree. C. for a period of 10 minutes to 5 hours in a non-reducing atmosphere.
Catalysts comprising rhenium heptoxide deposited on an alumina can also be modified by addition of an oxide of another metal. These modified catalysts contain, for example, rhenium as an oxide, 0.01% to 20% by weight expressed as rhenium metal, deposited on a support containing at least 75% by weight of alumina and 0.01% to 30% by weight of at least one oxide of a metal selected from the group formed by niobium and tantalum, as described in French patent FR-A-2 709 125.
The metathesis reaction is preferably carried out in the liquid phase, in the absence of oxygen, oxygen-containing compounds and moisture, and at a temperature in the range 0.degree. C. to 200.degree. C., preferably in the range 20.degree. C. to 150.degree. C., at a pressure at least equal to that of the vapour tension of the reaction mixture at the reaction temperature.
The catalyst can be used in a fixed bed. However, since it must be regenerated frequently, it would then be necessary to use at least two reactors in parallel, one in operational mode while the other is in regenerational mode. Preferably, a catalytic mobile bed system as described in French patent FR-A-2 608 595 is used. The catalyst is extracted at regular intervals from the bottom of the reactor and transferred continuously to a regeneration system, from which it is sent to the top of the reactor.
Because of thermodynamic limitations, unconverted ethylene is fractionated in a first distillation column (D1) and recycled to the metathesis reactor. A second distillation column (D2) separates the propylene and a third column (D3) separates the unconverted C.sub.4 hydrocarbons which can be recycled to the metathesis reactor, also a small quantity of a heavier fraction.